Light emitting device having a plurality of light emitting cells and method of fabricating the same

ABSTRACT

Disclosed is a light emitting device having a plurality of light emitting cells. The light emitting device comprises a thermally conductive substrate, such as a SiC substrate, having a thermal conductivity higher than that of a sapphire substrate. The plurality of light emitting cells are connected in series on the thermally conductive substrate. Meanwhile, a semi-insulating buffer layer is interposed between the thermally conductive substrate and the light emitting cells. For example, the semi-insulating buffer layer may be formed of AlN or semi-insulating GaN. Since the thermally conductive substrate having a thermal conductivity higher than that of a sapphire substrate is employed, heat-dissipating performance can be enhanced as compared with a conventional sapphire substrate, thereby increasing the maximum light output of a light emitting device that is driven under a high voltage AC power source. In addition, since the semi-insulating buffer layer is employed, it is possible to prevent an increase in a leakage current through the thermally conductive substrate and between the light emitting cells.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of prior application Ser. No.11/815,255, filed Aug. 1, 2007, which is the National Stage ofInternational Application No. PCT/KR2005/002124, filed on Jul. 5, 2005,and claims priority to and the benefit of Korea Patent Application No.10-2005-0010349, filed on Feb. 4, 2005, Korea Patent Application No.10-2005-0010978, filed on Feb. 5, 2005, and Korea Patent Application No.10-2005-0023185, filed on Mar. 21, 2005, which are all herebyincorporated by reference for all purposes as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light emitting device and a method offabricating the same, and more particularly, to a light emitting devicehaving a plurality of light emitting cells serially connected in asingle chip, and a method of fabricating the same.

2. Discussion of the Background

A light emitting diode is a photoelectric transformation semiconductorelement in which an N-type semiconductor and a P-type semiconductor arejoined together to form a junction and electrons and holes arere-combined to emit light. Such light emitting diodes are widely used asa display device and a backlight. In addition, the light emitting diodeconsumes less electric power as compared with conventional in-candescentbulbs or fluorescent lamps, and has an extended service life, therebybroadening its applications to general illumination purposes whilereplacing the conventional incandescent bulbs and fluorescent lamps.

The light emitting diode alternates between on- and off-states under anAC power source, depending on the direction of a current appliedthereto. Therefore, if the light emitting diode is connected directly toan AC power source, there are problems in that it cannot emit light in acontinuous mode and may be easily damaged due to a reverse current.

As an attempt to solve the above problems in the art, Sakai et al.proposed a light emitting diode capable of being connected directly to ahigh voltage AC power source, which is disclosed in InternationalPublication No. WO 2004/023568A1 entitled “Light-Emitting Device HavingLight-Emitting Elements.”

In the publication WO 2004/023568A1, LEDs are serially connected in atwo-dimensional pattern on an insulation substrate such as a sapphiresubstrate to form LED arrays. Two LED arrays are connected in reverseparallel on the sapphire substrate. Consequently, a single chip lightemitting device that can be driven by the AC power source is provided.

However, since the sapphire substrate has a relatively low thermalconductivity, heat cannot be smoothly dissipated. This limited heatdissipation leads to a limited maximum light output of the lightemitting device. Therefore, there is a need for a continuous effort toimprove the maximum light output of a light emitting device under a highvoltage AC power source.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an improved lightemitting device of which the maximum light output can be increased undera high voltage AC power source.

Another object of the invention is to provide a light emitting devicehaving a plurality of light emitting cells serially connected to eachother, wherein an increase in a leakage current through the substratecan be avoided even when an electrically conductive substrate is used.

A further object of the invention is to provide a method of fabricatingan improved light emitting device of which the maximum light output canbe increased under a high voltage AC power source without any increasein a leakage current through a substrate.

In order to achieve these objects, the present invention discloses alight emitting device having a plurality of light emitting cells. Alight emitting device according to an aspect of the present inventioncomprises a thermally conductive substrate having a thermal conductivityhigher than that of a sapphire substrate. A plurality of light emittingcells are connected in series on the thermally conductive substrate.Meanwhile, a semi-insulating buffer layer is interposed between thethermally conductive substrate and the light emitting cells. Since thethermally conductive substrate having a thermal conductivity higher thanthat of a sapphire substrate is employed, heat-dissipating performancecan be improved as compared with a conventional sapphire substrate,thereby increasing the maximum light output of a light emitting devicedriven under a high voltage AC power source. In addition, since thesemi-insulating buffer layer is used, it is possible to prevent anincrease in a leakage current through the thermally conductive substrateand between the light emitting cells.

The term “thermally conductive substrate” used herein means a substrateof a material having a thermal conductivity higher than that of asapphire substrate. In addition, the term “semi-insulating” materialmeans a material with high resistance, which generally has a resistivityof 10⁵ Ωcm or higher and includes insulation materials unlessparticularly specified.

The thermally conductive substrate may be an AlN or SiC substrate. Inaddition, the SiC substrate may be a semi-insulating or N-type SiCsubstrate. The AlN and SiC substrates have a thermal conductivity whichis about ten or more times as high as that of a sapphire substrate.Thus, the AlN or SiC substrate can provide a light emitting devicehaving improved heat-dissipating performance as compared with a lightemitting device employing a sapphire substrate.

The semi-insulating buffer layer may be formed of undoped AlN. The AlNhas an appropriate crystal structure between the SiC substrate and agroup III-nitride.

Alternatively, the semi-insulating buffer layer may be formed ofsemi-insulating GaN. The semi-insulating GaN may include undoped GaN orGaN doped with an acceptor. In general, undoped GaN exhibits propertiesof a semi-insulating or N-type semiconductor depending on the kind ofsubstrate. In case where undoped GaN exhibits properties of an N-typesemiconductor, semi-insulating GaN can be provided by doping this GaNwith an acceptor.

The acceptor may be an alkaline metal, an alkaline earth metal, or atransition metal, particularly, iron (Fe). Fe can be employed to grow asemi-insulating GaN buffer layer, without affecting the structuralproperties of GaN.

Each of the light emitting cells includes an N-type semiconductor layer,an active layer and a P-type semiconductor layer. The N-typesemiconductor layers and the P-type semiconductor layers of adjacentlight emitting cells are electrically connected to one another by meansof metallic wiring.

A light emitting device according to another aspect of the presentinvention includes a semi-insulating substrate. The semi-insulatingsubstrate may be an AlN or SiC substrate. A plurality of light emittingcells are serially connected to one another on the semi-insulatingsubstrate. According to this aspect, since the plurality of lightemitting cells are formed directly on the AlN or semi-insulating SiCsubstrate, the fabricating process thereof can be simplified.

The present invention also discloses a method of fabricating a lightemitting device having a plurality of light emitting cells. The methodof the present invention comprises preparing a thermally conductivesubstrate having a thermal conductivity higher than that of a sapphiresubstrate. A semi-insulating buffer layer is formed on the thermallyconductive substrate, and an N-type semiconductor layer, an active layerand a P-type semiconductor layer are formed on the semi-insulatingbuffer layer. Thereafter, the P-type semiconductor layer, the activelayer and the N-type semi-conductor layer are patterned to form aplurality of light emitting cells, each of which the N-typesemiconductor layer is partially exposed. Then, metallic wiring isformed to serially connect the light emitting cells in such a mannerthat the N-type semiconductor layer of each light emitting cell isconnected to the P-type semiconductor layer of a light emitting celladjacent thereto. Accordingly, it is possible to fabricate an improvedlight emitting device that can increase the maximum light output thereofunder a high voltage AC power source.

The thermally conductive substrate may be an AlN substrate, or asemi-insulating or N-type SiC substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view illustrating a light emitting device having aplurality of light emitting cells according to an embodiment of theinvention.

FIG. 2 is a circuit diagram illustrating a light emitting deviceincluding a bridge rectifier according to an embodiment of theinvention.

FIGS. 3 to 7 are sectional views illustrating a method of fabricating alight emitting device having a plurality of light emitting cellsaccording to an embodiment of the invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. The followingembodiments are provided only for illustrative purposes, so that thoseskilled in the art can fully understand the principle of the presentinvention. Thus, the present invention is not limited to the embodimentsset forth herein but may be implemented in many differentconfigurations. In the drawings, the width, length, thickness and thelike of each component may be exaggerated for convenience and clarity ofillustration. Like reference numerals indicate like elements throughoutthe description.

FIG. 1 is a sectional view illustrating a light emitting device with aplurality of light emitting cells serially connected to one anotheraccording to an embodiment of the invention.

Referring to FIG. 1, the light emitting device of the present inventioncomprises a thermally conductive substrate 110, a buffer layer 120formed on the substrate 110, a plurality of light emitting cells 100-1to 100-n patterned on the buffer layer 120, and metallic wiring 170-1 to170-n-1 for serially connecting the plurality light emitting cells 100-1to 100-n to each other.

The thermally conductive substrate 110 is a substrate of a material thathas a thermal conductivity relatively higher than that of a sapphiresubstrate. The thermally conductive substrate 110 may be an AlNsubstrate or a semi-insulating or N-type SiC substrate.

The buffer layer 120 is used for alleviating lattice-mismatch betweenthe substrate 110 and semiconductor layers to be formed thereon. Inaddition, in some embodiments of the present invention, the buffer layer120 may be employed to insulate the light emitting cells 100-1 to 100-nfrom the substrate 110. Furthermore, the light emitting cells should beelectrically isolated from one another. Thus, the buffer layer 120 isformed of a semi-insulating material film. In a case where the substrate110 is a semi-insulating AlN substrate or semi-insulating SiC substrate,the buffer layer 120 may be omitted.

In this embodiment, the semi-insulating buffer layer 120 may be an AlNor semi-insulating GaN layer. Since undoped AlN generally has aninsulation property, undoped AlN may be used as the AlN. Meanwhile,undoped GaN generally exhibits properties of an N-type semiconductor ora semi-insulation property, depending on the growth process and thesubstrate material. Therefore, in case where undoped GaN has asemi-insulation property, undoped GaN becomes the semi-insulating GaN.On the other hand, if the undoped GaN exhibits the properties of anN-type semiconductor, doping is made with an acceptor for compensation.The acceptor may include an alkaline metal, an alkaline earth metal, ora transition metal, particularly, iron (Fe) or chromium (Cr).

A method of forming a semi-insulating GaN layer on a sapphire substrateis disclosed in “Growth of Fe doped semi-insulating GaN by metalorganicchemical vapor deposition” by Heikman et al., in Applied Physics Letterspublished on Jul. 15, 2002. Heikman et al. used an MOCVD technique usingferrocene (Cp₂Fe) as a precursor in order to form a semi-insulating GaNlayer on a sapphire substrate.

Generally, undoped GaN formed on a sapphire substrate using the MOCVDtechnique is N-type GaN. This is because remaining oxygen atoms act asdonors in a GaN layer. Thus, doping is made with a metallic materialsuch as Fe, which acts as an acceptor, to compensate the donor, therebyforming semi-insulating GaN.

The technique by which doping is made with an acceptor to form asemi-insulating GaN can be identically applied in the embodiments of thepresent invention. For example, undoped GaN formed on a SiC substratecan become N-type GaN by impurities such as Si. Thus, thesemi-insulating GaN buffer layer 120 can be formed by performing dopingwith a metallic material such as Fe. Here, the entire thickness of thesemi-insulating buffer layer 120 is not necessarily required to be dopedwith the acceptor. The doping may be made with an acceptor such as Fe tothe partial thickness of the buffer layer 120.

Also, the acceptor can be doped by an ion implantation technique.

The semi-insulating buffer layer 120 may be continuous between the lightemitting cells 100-1 to 100-n, as illustrated, but may be discretetherebetween.

Meanwhile, each of the light emitting cells 100-1 to 100-n includes a PNjunction nitride semiconductor layer.

In this embodiment, each of the light emitting cells includes an N-typesemi-conductor layer 130, an active layer 140 formed on a predeterminedarea of the N-type semiconductor layer 130, and a P-type semiconductorlayer 150 formed on the active layer 140. An upper surface of the N-typesemiconductor layer 130 is exposed at least partially. Ohmic metal layer160 and 165 may be formed on the N-type semiconductor layer 130 and theP-type semiconductor layer 150. In addition, an N-type semi-conductortunneling layer with a high concentration of 1×10¹⁹˜1×10²²/cm³ orsemi-metal layer may be formed on the N-type semiconductor layer 130 orthe P-type semi-conductor layer 150. A transparent electrode layer (notshown) may also be further formed on the N-type semiconductor tunnelinglayer or semi-metal layer.

The N-type semiconductor layer 130 is an N-type Al_(x)In_(y)Ga_(1-x-y)N(0≦x, y≦1) film and may include an N-type clad layer. In addition, theP-type semiconductor layer 150 is a P-type Al_(x)In_(y)Ga_(1-x-y)N (0≦x,y≦1) film and may include a P-type clad layer.

The N-type semiconductor layer 130 can be formed through silicon-doping,and the P-type semiconductor layer 150 may be formed through zinc- ormagnesium-doping.

The active layer 140 is a region where electrons and holes arerecombined and includes InGaN. Depending on the material forming theactive layer 140, the wavelength of light emitted from thelight-emitting cell is determined. The active layer 140 may be amulti-layered film in which quantum-well layers and barrier layers arealternately formed. The barrier layer and the quantum-well layer may bebinary to quaternary compound semiconductor layers which can beexpressed in a general formula Al_(x)In_(y)Ga_(1-x-y)N (0≦x,y,x+y≦1).

The light emitting cells are connected in series by means of themetallic wiring 170-1 to 170-n-1. In this embodiment, a number of lightemitting cells 100-1 to 100-n that can be driven by an AC power sourceare connected in series by means of the metallic wiring. That is, thenumber of serially connected light-emitting cells 100 is limited by anAC driving voltage/current applied to the light emitting device and avoltage required to drive a single light-emitting cell. For example, inthe case of a 3.3V-driven light emitting cell, about 67 cells can beserially connected under an AC voltage of 220V. In addition,approximately 34 cells can be connected in series under an AC voltage of110V.

As illustrated in FIG. 1, in the light emitting device where n lightemitting cells 100-1 to 100-n are serially connected, the N-typesemiconductor layer 130 of the first light emitting cell 100-1 and theP-type semiconductor layer 150 of the second light emitting cell 100-2are connected through the first metallic wiring 170-1. The N-typesemiconductor layer 130 of the second light emitting cell 100-2 and theP-type semi-conductor layer (not shown) of the third light emitting cell(not shown) are connected through the second metallic wiring 170-2.Moreover, the N-type semiconductor layer (not shown) of the (n-2)-thlight emitting cell (not shown) and the P-type semi-conductor layer 150of the (n-1)-th light emitting cell 100-n-1 are connected through the(n-2)-th metallic wiring 170-n-2. The N-type semiconductor layer 130 ofthe (n-1)-th light emitting cell 100-n-1 and the P-type semiconductorlayer 150 of the n-th light emitting cell 100-n are connected throughthe (n-1)-th metallic wiring 170-n-1.

The above serially connected light emitting cells constitute an LEDarray, as disclosed in International Publication No. WO 2004/023568A1.In the meantime, the light emitting device may have two LED arrays,which are connected to each other in reverse-parallel, to be used forillumination under an AC power source. The P-type semiconductor layer150 of the first light emitting cell 100-1 and the N-type semi-conductorlayer 130 of the n-th light emitting cell 100-n may be formed with aP-type pad and an N-type pad (not shown) for electrical connection tothe AC power source, respectively.

Alternatively, the light emitting device may include a bridge rectifierconsisting of diodes in order to rectify an AC current. FIG. 2 shows acircuit diagram of a light emitting device including a bridge rectifier.

Referring to FIG. 2, an LED array 41 includes light emitting cells 41 a,41 b, 41 c, 41 d, 41 e and 41 f serially connected to one another. Abridge rectifier comprising diodes D1, D2, D3 and D4 is disposed betweenan AC power source 45 and the LED array 41 and between a ground and theLED array 41. The diodes D1, D2, D3 and D4 may be formed by means of thesame process as of the light emitting cells. That is, the bridgerectifier can be fabricated with the light emitting cells.

An anode terminal of the LED array 41 is connected to a node between thediodes D1 and D2, and a cathode terminal thereof is connected to a nodebetween the diodes D3 and D4. Meanwhile, a terminal of the AC powersource 45 is connected to a node between the diodes D1 and D4, and theground is connected to a node between the diodes D2 and D3.

If the AC power source 45 has a positive phase, the diodes D1 and D3 ofthe bridge rectifier are turned on and the diodes D2 and D4 are turnedoff. Thus, a current flows to the ground via the diode D1 of the bridgerectifier, the LED array 41, and the diode D3 of the bridge rectifier.

On the other hand, if the AC power source 45 has a negative phase, thediodes D1 and D3 of the bridge rectifier are turned off and the diodesD2 and D4 thereof are turned on. Thus, the current flows to the AC powersource via the diode D2 of the bridge rectifier, the LED array 41, andthe diode D4 of the bridge rectifier.

Consequently, the connection of the bridge rectifier to the LED array 41enables the AC power source 45 to continuously drive the LED array 41.The terminals of the bridge rectifier are configured to be connected tothe AC power source 45 and the ground, as shown in FIG. 2. But, theterminals of the bridge rectifier may be configured to be connected toboth terminals of the AC power source.

According to this embodiment, a single LED array can be driven by beingelectrically connected to an AC power source, thereby improving theefficiency of use of the LED array.

Next, a method of fabricating a light-emitting device having a pluralityof light emitting cells will be described.

FIGS. 3 to 7 are sectional views illustrating a method of fabricating alight emitting device having a plurality of light emitting cellsaccording to an embodiment of the invention.

Referring to FIG. 3, a semi-insulating buffer layer 120 is formed on athermally conductive substrate 110. The thermally conductive substrate110 may be an AlN or SiC substrate. In addition, the SiC substrate maybe a semi-insulating or N-type substrate.

In general, a single crystal SiC substrate exhibits properties of anN-type semi-conductor. It is known that this is because nitrogencontained in the SiC substrate acts as an donor. Thus, doping is madewith an acceptor such as vanadium (V) to grow a semi-insulating SiCsingle crystal. Meanwhile, U.S. Pat. No. 6,814,801 discloses a method ofgrowing a semi-insulating SiC crystal without doping with vanadium.These techniques can be used to provide a semi-insulating SiC substrate.On the other hand, an acceptor such as Fe, V, C or Si can be injectedinto a SiC substrate using an ion implantation technique to convert theupper part of the SiC substrate into a semi-insulating SiC layer 112.

The semi-insulating buffer layer 120 is formed through a metalorganicchemical vapor deposition (MOCVD) method, molecular beam epitaxy (MBE)method, a hydride vapor phase epitaxy (HVPE) method, or the like. Thebuffer layer 120 may be an AlN or semi-insulating GaN layer. Thesemi-insulating GaN layer may be an undoped GaN layer or a GaN layerdoped with an acceptor. The acceptor may be an alkaline metal, analkaline earth metal or a transition metal, particularly, iron (Fe) orchromium (Cr). The acceptor can be doped using a deposition techniquewith precursors during the formation of the GaN layer, or using an ionimplantation technique after the formation of the GaN layer.

In the case of using an N-type SiC substrate, the semi-insulating bufferlayer 120 electrically insulates the light emitting cells from theN-type SiC substrate to prevent a leakage current through the substrate.On the other hand, in the case of using a semi-insulating SiC substrate,the process of forming the semi-insulating buffer layer 120 can beomitted.

Referring to FIG. 4, an N-type semiconductor layer 130, an active layer140 and a P-type semiconductor layer 150 are formed on thesemi-insulating buffer layer 120. These semiconductor layers 130, 140and 150 can be formed continuously within one processing chamber. TheN-type semiconductor layer 130, the active layer 140 and the P-typesemiconductor layer 150 can be formed using the MOCVD method, the MBEmethod, or the HVPE method, and each of the layers may be formed in amulti-layer. An N-type semiconductor tunneling layer with a highconcentration of 1×10¹⁹˜1×10²²/cm³ or semi-metal layer may be formed onthe N-type semiconductor layer 130 and/or the P-type semiconductor layer150, a transparent electrode layer (not shown) may also be furtherformed on the N-type semiconductor tunneling layer or semi-metal layer.

Referring to FIG. 5, the P-type semiconductor layer 150, the activelayer 140 and the N-type semiconductor layer 130 are patterned to formdiscrete light emitting cells 100-1 to 100-n. The layers can bepatterned using a photolithographic and etching technique. For example,a photoresist pattern is formed on the P-type semiconductor layer 150,and the P-type semiconductor layer 150, the active layer 140 and theN-type semiconductor layer 130 are then etched in sequence using thephotoresist pattern as an etching mask. Accordingly, the discrete lightemitting cells are formed. At this time, the semi-insulating bufferlayer 120 may be etched to expose the substrate 110.

Referring to FIG. 6, an upper surface of the N-type semiconductor layer130 is partially exposed by patterning the P-type semiconductor layer150 and the active layer 140 of each of the discrete light emittingcells 100-1 to 100-n. This patterning process can be carried out using aphotolithographic and etching technique. That is, a photoresist patternis formed on the substrate 110 having the discrete light emitting cells100-1 to 100-n, and the P-type semiconductor layer 150 and the activelayer 140 are partially etched using the photoresist pattern as anetching mask. Consequently, the etched portions of the P-typesemiconductor layer and the active layer enable the partial exposure ofthe N-type semiconductor layer 130.

This etching process can be performed through a wet or dry etchingprocess. The dry etching process may be a plasma dry etching process.

Referring to FIG. 7, a P-type ohmic metal layer 160 and an N-type ohmicmetal layer 165 are formed on the P-type semiconductor layer 150 and theN-type semiconductor layer 130, respectively.

The ohmic metal layers 160 and 165 can be formed by opening regionswhere the ohmic metal layers 160 and 165 will be formed using aphotoresist pattern (not shown) and performing a metal depositionprocess. The P-type ohmic metal layer 160 and the N-type ohmic metallayer 165 may be formed through the same process or separate respectiveprocesses. The ohmic metal layers 160 and 165 may be formed of at leastone selected from Pb, Sn, Au, Ge, Cu, Bi, Cd, Zn, Ag, Ni and Ti.

Thereafter, the N-type ohmic metal layers 165 and the P-type ohmic metallayers 160 of adjacent light emitting cells are connected using metallicwiring 170-1 to 170-n-1. The metallic wiring can be formed through anair bridge process or a step-cover process.

The air bridge process is disclosed in International Publication No. WO2004/023568A1 and will be briefly described. First, a first photoresistpattern with openings through which portions of the ohmic metal layers160 and 165 are exposed is formed on the substrate provided with thelight emitting cells and the ohmic metal layers 160 and 165. Then, athin metallic layer is formed using an e-beam evaporation technique. Themetallic layer is formed over the openings and the entire upper surfaceof the first photoresist pattern. Thereafter, a second photoresistpattern is formed to expose regions between adjacent light emittingcells to be connected, and the metallic layer on the openings.Subsequently, gold or the like is plated and the first and secondphotoresist patterns are then removed using a solution such as asolvent. As a result, only wiring for connecting adjacent light emittingcells is left, and the remaining metallic layer and the photoresistpatterns are fully removed.

On the other hand, the step-cover process includes forming an insulationlayer on the substrate having the light emitting cells and the ohmicmetal layers. The insulation layer is patterned using aphotolithographic and etching technique to form openings through whichthe ohmic metal layers 160 and 165 on the P- and N-type semiconductorlayers are exposed. Then, an e-beam evaporation technique is employed tofill the openings and form a metallic layer for covering the insulationlayer. Thereafter, the metallic layer is patterned using aphotolithographic and etching technique to form wiring for connectingneighboring light emitting cells to each other. Various modifications tothis step-cover process can be made. When the step-cover process isused, the insulation layer supports the wiring, thereby improving thereliability of the wiring.

Thereafter, a P-type pad and an N-type pad for electrical connection toan AC power source are formed at the light emitting cells 100-1 and100-n located at both distal ends.

Although the light emitting cells are illustrated as being arranged in astraight line in the drawings, this is only to ensure convenience ofexplanation. The light emitting cells may be arranged on a plane invarious patterns and configurations, as shown in InternationalPublication No. WO 2004/023568A1.

According to the present invention, the heat-dissipating performance ofa light emitting device can be enhanced. Thus, it is possible to providea light emitting device having increased maximum output under a highvoltage AC power source. Furthermore, it is possible to provide a lightemitting device having a plurality of light emitting cells and employinga semi-insulting substrate or a semi-insulating buffer layer to preventan increase in a leakage current through the substrate.

1. A light emitting device having a plurality of light emitting cells,comprising: a thermally conductive substrate having a thermalconductivity higher than that of a sapphire substrate; a plurality oflight emitting cells serially connected to one another on the thermallyconductive substrate; and a semi-insulating buffer layer interposedbetween the thermally conductive substrate and the light emitting cells,wherein the thermally conductive substrate comprises AlN or an N-typeSiC substrate.
 2. The device of claim 1, wherein the semi-insulatingbuffer layer is formed of undoped AlN.
 3. The device of claim 1, whereinthe semi-insulating buffer layer is formed of semi-insulating GaN. 4.The device of claim 3, wherein the semi-insulating GaN is GaN doped withan acceptor.
 5. The device of claim 1, wherein each of the lightemitting cells comprises an N-type semiconductor layer, an active layer,and a P-type semi-conductor layer, and the N-type semiconductor layerand the P-type semi-conductor layer of adjacent light emitting cells areelectrically connected in series by means of metallic wiring.
 6. Thedevice of claim 1, wherein the semi-insulating buffer layer is partiallydoped with an acceptor.
 7. The device of claim 1, wherein thesemi-insulating buffer layer is continuously disposed between each ofthe light emitting cells.
 8. A method of fabricating a light emittingdevice, comprising: forming a thermally conductive substrate comprisingAlN or an N-type SiC substrate; forming a semi-insulating buffer layeron the thermally conductive substrate; forming an N-type semiconductorlayer, an active layer, and a P-type semi-conductor layer on thesemi-insulating buffer layer; patterning the P-type semiconductor layer,the active layer, and the N-type semi-conductor layer to form aplurality of light emitting cells, each of the light emitting cellshaving a partially exposed N-type semiconductor layer; and formingmetallic wiring serially connecting the light emitting cells in such away that the N-type semiconductor layer of each of the light emittingcells is connected to the P-type semiconductor layer of a light emittingcell adjacent thereto.
 9. The method of claim 8, wherein thesemi-insulating buffer layer is formed of undoped AlN.
 10. The method ofclaim 8, wherein the semi-insulating buffer layer is formed ofsemi-insulating GaN.
 11. The method of claim 10, wherein thesemi-insulating GaN is GaN doped with an acceptor.
 12. The method ofclaim 11, wherein the acceptor is doped using an ion implantationtechnique.
 13. The method of claim 12, wherein the ion implantationtechnique dopes the acceptor to a partial thickness of thesemi-insulating GaN.